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Winds of change in middle-of-the-road galaxy
Posted: 4 March 2010

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A howling gale of radiation blowing from a black hole in the spiral galaxy M77 is impacting on star formation despite the black hole’s ‘moderate’ size, according to observations from the Chandra X-ray Observatory. The findings show that even less active, smaller black holes can have an effect on the evolution of their hosts.

A multi-wavelength image of M77, combining visible data (green) from the Hubble Space Telescope with radio data (blue) from the dishes of the Very Large Array in New Mexico, USA, and X-ray observations (red) from Chandra. The radio data actually reveals the winds in the shape of a modest jet, while glowing red gas clouds surround the core. Image: NASA/CXC/MIT/C Canizares/D Evans et al/STScI/NSF/NRAO/VLA.

“We have shown that even these middle-of-the-road black holes can pack a punch,” says Dan Evans of the Massachusetts Institute of Technology, who led the Chandra observations and presented the findings this week at a meeting of the High Energy Astrophysical Division of the American Astronomical Society in Hawaii. The black hole in M77 measures in at twice the mass of the Milky Way’s own black hole, Sagittarius A* (which has a mass of 4.3 million solar masses).

It has been known for some time that the most powerful black holes with masses hundreds or thousands of times greater than those in the Milky Way or M77 can have a startling effect on their host galaxies, heating gas or even blowing the gas out of the galaxy entirely. The winds are whipped up by magnetic fields in gas that is spiralling around the black hole, driving some of the gas outwards, away from the black hole’s grasp. At their most extreme black hole winds are seen as jets blasting outwards at almost the speed of light, as in the giant elliptical galaxy M87. The winds in M77 are much slower, ‘merely’ one and a half million kilometres per hour, but are still powerful enough to cause heating of the surrounding gas. Hot gas becomes too energetic to collapse and form stars, and so if the feedback from a black hole is strong enough it can effectively put an end to star formation within a galaxy.

Chandra observed M77, which is about 50 million light years away, for five days with its High Energy Transmission Grating Spectrometer. It observed the black hole wind driving several solar masses of gas away from M77’s central regions every year, which is heating outlying gas clouds 3,000 light years from the black hole, causing them to emit X-rays and make themselves visible to Chandra.

The black hole in our own Milky Way does not have any kind of wind akin to the one in M77, and star-formation seems to be thriving in its vicinity. However, whereas Sagittarius A* may be quiet now, should it become active it could dramatically alter the inner regions of our Galaxy. Says Evans, “In the future, our own Galaxy’s black hole may undergo similar activity, helping to shut down the growth of new stars in the central region of the Milky Way.”